1. Field of the Invention
The present invention relates to a control device for an internal combustion engine which is able to use an alcohol mixed fuel in which alcohol is mixed with a hydrocarbon fuel and which is able to inject the alcohol mixed fuel from a port injection valve for injecting fuel to an induction port and from a cylinder injection valve for directly injecting fuel into a cylinder.
2. Description of Related Art
As an automotive internal combustion engine, there has been known an internal combustion engine for Flexible Fuel Vehicle (FFV) that is able to use a fuel in which alcohol such as ethanol is mixed with a hydrocarbon fuel such as gasoline. Further, there has been known a dual-injection internal combustion engine which includes a port injection valve for injecting fuel to an induction port and a cylinder injection valve for directly injecting fuel into a cylinder and which is able to change an injection ratio between these injection valves according to an operational state. Japanese Patent Application Publication No. 2009-216005 (JP 2009-216005 A) describes a technique related to a dual-injection internal combustion engine for FFV that has features of both of the above two types of internal combustion engines, that is, an internal combustion engine which includes a cylinder injection valve and a port injection valve and which is able to inject an alcohol mixed fuel from both of the injection valves.
In a conventional internal combustion engine for FFV, an engine control is performed by a method according to an alcohol concentration of a fuel. More specifically, as the alcohol concentration of the fuel is higher, a calorific value per unit volume becomes smaller. In view of this, a fuel injection amount is determined according to the alcohol concentration of the fuel. This is because if the fuel injection amount becomes excessive, emission increases, but if the fuel injection amount is insufficient, drivability becomes worse.
In the meantime, in a case of fuel injection by the cylinder injection valve, a fuel injection period during which stable burning is obtained is limited. Accordingly, in a case where the fuel injection amount from the cylinder injection valve is just increased according to the alcohol concentration of the fuel, the burning may become unstable.
Further, there are several types of commercially available alcohol mixed fuels of different alcohol concentrations. In view of this, when refueling is performed in a vehicle, there is such a possibility that a fuel having an alcohol concentration different from that of a currently used fuel is supplied to a fuel tank. In that case, an alcohol concentration in the fuel tank changes according to a fuel supply amount. However, a fuel that has been already drawn up from the fuel tank before the refueling, that is, a residual fuel in a fuel supply line keeps an alcohol concentration before the refueling. As a result, the fuel having the same alcohol concentration as the fuel before the refueling is injected from a fuel injection valve for a while after the refueling, and then, a fuel of which the alcohol concentration is changed by the refueling is injected from the fuel injection valve.
Here, the aforementioned dual-injection internal combustion engine includes a cylinder injection valve and a port injection valve for each cylinder. Further, the cylinder injection valve and the port injection valve are individually provided with respective fuel supply lines for distributing fuel to these fuel injection valves in each cylinder. According to such a configuration, while the fuel before the refueling is being replaced with the fuel after the refueling, fuels having different alcohol concentrations may be injected from the cylinder injection valve and the port injection valve in some cases.
For example, in the conventional internal combustion engine for FFV, on the premise that fuels having the same alcohol concentration are injected from the cylinder injection valve and the port injection valve, respective fuel injection amounts from these injection valves are determined according to respective blowing rates of the injection valves relative to a total amount of the fuels. In such a case, if there is a difference in the alcohol concentration between the fuel injected from the port injection valve and the fuel injected from the cylinder injection valve, the fuel injection amounts do not become amounts according to a desired air-fuel ratio, which conceivably causes a variation in the air-fuel ratio.